The views that summarize and display the information stored in the base tables of the
data dictionary. These views decode the base table data into useful information, such
as user or table names, using joins and WHERE clauses to simplify the information.
Most users are given access to the views rather than the base tables.

数据字典结构之用户可访问视图
1. 将基表的数据通过连接，where条件将基表中有用的信息转换展现给用户